This invention relates to a fiber optic star coupler and to an arrangement for interconnecting multiple circuit cards mounted in a rack or cabinet using such a coupler.
As recognized by Coyne, "Distributive Fiber Optic Couplers Using Rectangular Light Guides as Mixing Elements", Proceedings of the 2nd International Fiber Optics and Communications Exposition, 1979, a rectangular light guide provides a good implementation of a star coupler for interconnecting many terminals in a communications system. Using such a guide, nearly uniform intensity of light can be achieved over the area of the light guide output end. In addition, the rectangular geometry provides a configuration in which fibers can be arranged with low packing fraction loss. The light guide described by Coyne is a sandwich structure having a thick central layer of high refractive index core glass and at two opposed surfaces of the core glass, a thin layer of relatively low index cladding glass.
To make the outer surfaces of the glass block highly reflecting, they must be polished which is a time consuming operation. Moreover, using transparent cladding layers any light incident on the high index/low index interface at greater than the Brewster angle is lost from the star coupling block. The glass used in the construction of this star coupler must be of high quality to minimize light attenuation and to reduce the chance of long term refractive index change which would change the performance of the star coupling block.